4週5日目 せざるをえない
Perspective・object・obligation
接続：Vないstem＋ざるを得ない
forced to do it even though he didn't want to
注意 / Note：〜得る says objective ability; 〜かねない warns of a possible bad outcome.
比較 / Compare：Same article: 〜ないわけにはいかない／〜しかない. These models are often mistakenly chosen for each other — carefully read the 接続 and nuances above and then decide.
よく出る形 / Common form：動ない形の語幹＋ざるを得ない。Only する→せざるを得ない. Writing, hard. The voice is reluctant due to external circumstances.
計画を変更せざるを得ない。
Forced to change plans.
事情を説明せざるを得なかった。
I had to explain the situation.
資材が届かず、工事を延期せざるを得なかった。
Materials did not arrive in time so we were forced to postpone construction.